$events = Get-EventLog -LogName System
$events |
    Group-Object { $_.TimeGenerated.Date } |
    ForEach-Object {
        $sortedGroup = $_.Group | Sort-Object TimeGenerated
        [PSCustomObject]@{
            Date       = $_.Name
            FirstEvent = $sortedGroup[0].TimeGenerated
            LastEvent  = $sortedGroup[-1].TimeGenerated
        }
    }